Ventricular fibrillation is one of the most common life-threatening medical conditions that occurs with respect to the human heart. In ventricular fibrillation, the human heart's electrical activity becomes unsynchronized, which results in a loss of its ability to contract. As a result, a fibrillating heart immediately loses its ability to pump blood into the circulation system. A common treatment for ventricular fibrillation is to apply an electric pulse to the heart that is strong enough to stop the unsynchronized electrical activity and give the heart's natural pacemaker a chance to reinitiate a synchronized rhythm. External defibrillation is the method of applying the electric pulse to the fibrillating heart through a patient's thorax.
Existing external cardiac defibrillators first accumulate a high-energy electric charge in an energy store, typically a capacitor. When a switching mechanism is activated, the stored energy is applied to the patient via electrodes positioned on the patient's thorax. The resultant discharge of the capacitor causes a large current pulse to be transferred through the patient.
Circuitry in a defibrillator may be used to alter a pulse's duration and direction of flow, thereby affecting the shape of the pulse. Common defibrillating pulse shapes include damped sine and truncated exponential waveforms. Both types of waveforms can have single or multiple phases. A biphasic truncated exponential waveform has two phases. In the first phase of the pulse, current flows in one direction. In the second phase, current flows in the opposite direction.
Regardless of the waveform used by a defibrillator, the defibrillation pulse applied to a patient contains a certain amount of energy. The defibrillator industry uses energy settings on a defibrillator's control panel to indicate the selected amount of energy that the defibrillation pulse should deliver to the patient.
In existing defibrillation practice, when it is necessary to apply a succession of defibrillation pulses to a patient, the operator generally increases the selected amount of energy to be delivered by each successive pulse. Higher energy defibrillation pulses are used until defibrillation occurs, or the highest available energy is used. The American Heart Association recommends that an energy level of 200 joules be set for a first defibrillation pulse, 200 or 300 joules for a second defibrillation pulse, and 360 joules for a third defibrillation pulse.
When a defibrillating pulse is applied to a patient, the pulse encounters a resistance to the flow of electrical current through the patient. The resistance of a patient's thorax to the flow of electrical current is called transthoracic impedance (TTI). The magnitude of current flowing through a patient is directly proportional to the magnitude of the voltage difference across the electrodes used to deliver the defibrillation pulse to the patient and inversely proportional to the patient's TTI.
External defibrillators are likely to encounter patients with a wide range of TTI values. Thus, one challenge that faces external defibrillator manufacturers is to design defibrillators that work well over a wide range of patient TTI values. While conventional defibrillators are often specified for and tested with 50 ohm loads, patient TTI can vary greatly in a range from 25 to 180 ohms. Average patient TTI in a hospital setting is about 80 ohms.
Defibrillator circuits which generate damped sine and truncated exponential pulses respond differently to variations in transthoracic impedance. Damped sine defibrillator impedance response is passive; that is, the response is determined entirely by the amount of capacitance, inductance, and resistance in the circuit. As impedance increases, defibrillating pulse duration increases and peak current decreases.
Several factors affect the shape of waveforms produced by truncated exponential defibrillators in response to different TTI values. Both the capacitance and resistance of the circuit determine passively how quickly the current drops after its initial peak. The active control of a switch that truncates the discharge determines the duration of each phase of the pulse. By design, pulse duration typically increases with increasing TTI values. This is done to allow additional time for energy delivery before the pulse is truncated.
Prior art defibrillators are calibrated for energy delivery at a single, specified load impedance, typically 50 ohms. However, as noted earlier, the TTI of many patients exceeds 50 ohms. As a result, the amount of energy actually delivered to a patient is different than the energy level selected by the operator. With damped sine waveforms, patients with TTI greater than 50 ohms receive higher energy than the energy level selected by the operator. With truncated exponential waveforms having fixed durations, patients with TTI greater than 50 ohms receive less energy than the selected energy level. The peak current delivered to patients also drops as patient TTI increases. Prior art defibrillators using truncated exponential waveforms typically adjust the duration of the waveforms (i.e., increased duration with increased impedance) to compensate for a decrease in energy delivered. However, partly because of a reduction in peak current produced in higher impedance patients, long duration truncated exponential waveforms may be less effective among high impedance patients. See, for example, the article "Transthoracic Defibrillation of Swine with Monophasic and Biphasic Waveforms," Circulation 1995, Vol. 92, p. 1634, in which the authors Gliner et al. acknowledge that, for a biphasic truncated exponential waveform, pulse durations exceeding 20 milliseconds are less effective.
Recognizing that patient TTI values affect the amount of current actually delivered to a patient, the prior art has proposed various techniques designed to compensate for varying patient impedance values. For example, U.S. Pat. No. 4,574,810 to Lerman discloses a defibrillator designed to provide a peak current using an amperes per ohm factor based on a measured patient transthoracic resistance. U.S. Pat. Nos. 4,771,781 and 5,088,489, also to Lerman, disclose current-based defibrillation methods. These prior art defibrillation methods include first determining the patient's transthoracic resistance and then discharging a capacitor with the intent that the peak current preselected by the operator as appropriate for attaining defibrillation is delivered. An article related to this prior art technique is found at Lerman et al., "Current-Based Versus Energy-Based Ventricular Defibrillation: A Prospective Study," Journal of the American College of Cardiology, November 1988, Vol. 12, No. 5, page 1259. U.S. Pat. No. 4,840,177 to Charbonnier et al. discloses a method and apparatus wherein a patient's measured impedance is normalized and multiplied by a desired current to yield a target charge level for an energy storage device to induce the desired current in the patient on discharge. These prior art techniques focus on delivering a desired current rather than tailoring a defibrillation pulse shape to optimally deliver a desired amount of energy.
Another prior art technique in which adjustments are made in response to measured patient TTI is disclosed by Kerber et al. in "Automated impedance-based energy adjustment for defibrillation: experimental studies," Circulation, January 1985, Vol. 71, No. 1, page 136; and "Energy, current, and success in defibrillation and cardioversion: clinical studies using an automated impedance-based method of energy adjustment," Circulation, May 1988, Vol. 77, No. 5, page 1038. In these articles, the authors address the problem of providing an adequate amount of energy for defibrillation to different patients having high and low transthoracic impedance. The authors suggest basing a first defibrillation pulse on a low energy value, such as 100 joules. The authors believe that delivery of such a pulse to a low impedance patient may be sufficient for defibrillation. To address higher impedance patients, the authors suggest measuring patient impedance momentarily before defibrillation to determine if the patient impedance exceeds an arbitrary value such as 70 ohms. If a patient impedance in excess of 70 ohms is detected, the authors suggest automatically increasing the selected amount of energy.
For example, if a 100-joule pulse is selected by an operator, a defibrillation pulse of 200 joules would be delivered if the patient's TTI exceeded 70 ohms. Likewise, if 150 joules or 200 joules is selected, the defibrillator would deliver 300 joules or 400 joules, respectively. The authors' proposal is based on a belief that low energy defibrillation pulses are insufficient to defibrillate high impedance patients and that applying a higher energy first pulse to such patients will avoid the need to apply multiple defibrillation pulses.
Delivering different levels of energy to patients having different TTI levels is also disclosed in U.S. Pat. No. 5,111,813 to Charbonnier et al. An impedance-dependent delivered energy is derived by multiplying a patient's TTI by a parameter E.sub.d /Z.sub.p measured in joules per ohm. Accordingly, patients having lower TTI will receive a lower amount of energy while higher TTI patients receiver higher energy. This technique is not concerned with delivering a selected amount of energy to patients across the range of potential patient impedance.
Another prior art technique for dealing with differing patient TTI involves detecting the patient's actual transthoracic impedance during delivery of the first defibrillation shock. In U.S. Pat. No. 4,328,808 to Charbonnier et al. an alarm is activated if the detected TTI exceeds a predetermined value. The alarm notifies the operator of the need to increase the selected amount of energy before applying another defibrillation pulse.
Other prior art techniques to compensate for different patient TTI have involved altering the duration of the defibrillation pulse. For example, U.S. Pat. No. 5,230,336 to Fain et al. discloses a method of selecting a suggested pulse width for a second defibrillation pulse based on system impedance measured during delivery of a first defibrillation pulse. U.S. Pat. Nos. 5,601,612 and 5,593,427, both to Gliner et al., describe on-the-fly adjustment to the duration of a defibrillation pulse to compensate for patient-to-patient impedance differences. U.S. Pat. No. 5,540,723 to Ideker et al. discloses methods and apparatus for treating cardiac arrhythmias wherein the duration of a defibrillation pulse is adjusted in accordance with a detected pulse signal time constant, of which patient impedance is one factor. U.S. Pat. No. 5,607,454 to Cameron et al. teaches on-the-fly adjustment to the relative duration of phases of a multiphasic waveform depending on a monitored parameter detected during delivery of the waveform to a patient. U.S. Pat. No. 5,534,015 to Kroll et al. discloses an implantable defibrillator that uses a resistance value measured across electrodes to dynamically control the duration of a first portion of a multiphasic waveform's first phase. Nevertheless, compensating for increased patient TTI by increasing the duration of the defibrillation pulse after delivery of the pulse has begun is not satisfactory because in cases where there is insufficient charge on the capacitor, the current delivered by such a defibrillation pulse is not sufficient to defibrillate the heart, especially when a monophasic or biphasic truncated exponential waveform is employed.
While the prior art has recognized that defibrillator pulse current, energy, and shape are related to patient TTI, the prior art has not suggested that, particularly for truncated exponential defibrillation pulses, the shape of the pulse, including peak amplitude and phase duration should be adjusted according to patient TTI measured prior to delivery to assure delivery of a chosen amount of energy. The present invention is directed to providing a defibrillation method and apparatus wherein the amplitude and duration of a defibrillation pulse are determined prior to delivery based on a measured patient TTI so that the energy conveyed by a defibrillation pulse to the patient is near or exceeds the selected level regardless of the patient's TTI.